The present invention relates to semiconductor manufacturing, and in particular, to a method and apparatus to control temperature in a rapid thermal processing (RTP) system.
One of the procedures commonly used during the manufacture of semiconductor devices is rapid thermal processing (RTP), for example for annealing of a semiconductor wafer. Several different RTP methods have been proposed, including those utilizing a variety of sources of heat, such as arc-lamps and heating blocks.
Because variations in wafer temperature during the annealing process can lead to yield problems in the resulting semiconductor devices, temperature is sought to be closely controlled and monitored during the RTP process. For example, the wafer is sought to be heated to a specified temperature for a specified time period, and the temperature conditions should not change from wafer to wafer. The specified times and temperatures often vary for the several different heating steps of a typical semiconductor production process.
Semiconductor manufacturers thus are continually in search of methods to ensure precise control of wafer temperature during annealing. Conventional RTP systems typically employ a combination of techniques for controlling temperature, including specialized heat sources, reflective or absorptive heating chambers, and wafer rotation.
A problem may arise in that the temperature of each wafer undergoing processing depends on the heat absorption characteristics of the wafer, which may vary significantly from wafer to wafer depending on the wafer type, films added to the wafer, and emissivity of the wafer surface. In addition, it is often difficult to directly measure the temperature of the wafer undergoing heating, due to the presence of various films and layers on the wafer and the desire to avoid invasive measurement methods. Thus, monitoring and control of wafer temperature presents a difficult problem.
One popular RTP system employs a heating block or xe2x80x9csusceptorxe2x80x9d of nearly constant temperature that is placed adjacent a wafer undergoing annealing. The temperature of the susceptor is closely monitored and controlled, and temperature variations in the wafer may be determined from temperature variations in the susceptor. This system offers several advantages over lamp-based systems, in that the wafer heating profile is less dependent upon wafer surface characteristics, e.g. backside films.
Temperature may be tightly controlled using such a susceptor-based system, yet wafer-to-wafer temperature variations may continue to occur. When a wafer is initially placed adjacent the susceptor, the temperature of the susceptor drops as heat is initially transferred to the wafer. Over time, the wafer heats up and the temperature of the susceptor, as well as the wafer, returns to a steady-state temperature.
During the initial heating of each wafer, the heat absorption characteristics of the wafer determine the rate of drop of the susceptor temperature and rate of increase of the wafer temperature. When a series of wafers is successively heated using this RTP system and each wafer has different heat absorption characteristics, wafer-to-wafer temperature variations may occur. As noted, this successive wafer-to-wafer temperature variation may result in possible yield problems for the produced semiconductor devices.
Accordingly, there is a strong desire and need to produce an RTP method that substantially eliminates wafer-to-wafer temperature variations for annealing of wafers having varying heat absorption characteristics.
An apparatus and method for rapid thermal processing (RTP) of semiconductor wafers is provided that compensates for variations in heat absorption characteristics of the wafers. The invention nearly eliminates wafer-to-wafer temperature variation by using a model of the heat absorption characteristics of different wafer types to predict a steady state temperature of a wafer undergoing processing. This prediction is used to detect potential variations in wafer temperature during the RTP process and correct for these variations by adjusting the output of the heat source.
In an exemplary embodiment of the invention, the rate at which the temperature of the heating block, or susceptor, changes when the wafer is initially placed in the heating chamber is measured and this measurement is used in the prediction of the steady-state temperature of the wafer. Heating values of the susceptor may be adjusted to achieve a desired steady-state wafer temperature. Measurements and adjustments may be stored and used to update the model of wafer heat absorption characteristics for later processing of wafers.